The present invention relates to light emitting diode (LED) printbars such as those used in xerographic or digital printer systems and, more particularly, to an improved apparatus and method for maintaining LED printbar uniformity and equalization throughout the operating life of the printbar.
Printbars which are used in image recording systems are well known in the art. Such printbars are generally comprised of a linear array of discrete, light-emitting sources. Light emitting diode (LED) printbars are commonly used because of their high resolution, which is obtained by arranging a large number of closely spaced LEDs in a linear array. By providing relative motion between the LED printbar and a photoreceptor, and by selectively energizing the LEDs at the proper times, a desired latent electrostatic image can be produced on the photoreceptor. The production of a desired latent image is usually performed by having each LED expose a corresponding pixel on the photoreceptor in accordance with image-defining video data information applied to the printbar through driver circuitry. Conventionally, digital data signals from a data source, which may be a Raster Input Scanner (RIS), a computer, or some other source of digitized image data, is clocked into a shift register. Some time after the start of a line signal, individual LED drive circuits are then selectively energized to control the on/off timing of currents flowing through the LEDs. The LEDs selectively turn on and off to form a line exposure pattern on the surface of the photoreceptor.
The LEDs of most LED printbars are arranged in a linear array of one or more rows. By making the length of a row as long as the image that is to be formed an LED printbar can produce a desired image line by line. Since it is difficult to produce a row of closely spaced LEDs with the required length (for example, 8 to 14 inches) LED chips of smaller lengths are usually butted together and interconnected to act as a single row. If more than one row is used for the LED printbar, the various rows are usually offset in a staggered fashion.
To create high quality images using an LED printbar, each of the LEDs should output the same amount of light when activated. To meet current copy or print quality goals, the printbar light output uniformity must be within plus or minus 1 or 2%. It is known in the prior art to correct printbar LED outputs to this uniformity level during an initial calibration procedure for the LEDs. A correction matrix of light output values for each pixel is created and stored in a memory for the printbar. Those values are downloaded to correction circuitry each time the printer is to be used. The correction circuitry can then compensate for light output differences by controlling an electrical signal, usually the drive current, to the individual LEDs based upon the stored correction values.
While the above initial calibration procedure of achieving light output uniformity is generally successful, the individual LEDs of an LED printbar may have different aging characteristics which can eventually result in unacceptable non-uniformity pixel-to-pixel exposure beyond 1 and 2 percent. One solution to this aging problem is to periodically scan the LED printbar with a photosensor as each LED is individually turned on. The light output from each LED is then measured and, if necessary, the stored correction value for the particular LED is updated to reflect changes in light output. While this system compensates for aging, it is rather expensive and uses valuable space near the photoreceptor.
It is an object of the present invention to provide sensing means to detect and compensate for non-uniform light output from a LED printbar.
It is another object of the present invention to provide more than one set of correction data to control the LED printbar to compensate for non-uniform light output from the LED printbar.
According to the present invention, uniformity of light output from an LED printbar is achieved by current sensing or image sensing of auxiliary LEDs on the LED printbar. Initial correction data for the LED printbar is determined and stored in a correction memory. The contents of the correction memory are used to control the illumination of each of the LEDs in the printbar. Periodically, a photodetector measures the light output from the auxiliary LEDs and a comparator will compare the auxiliary LED output power data. The comparator will signal for new correction data in the correction memory to be used to control the illumination of each of the LEDs in the printbar and compensate for aging by providing more uniform light output.
The current sensing method will have the driving current for the LED printbar digitalized by an A/D converter, then shuffled to drive the auxiliary LEDs.
The image sensing method will have a line of data for the LED printbar, scaled and quantized, then shuffled to drive the auxiliary LEDs.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.